In the field of rotating electrical machines, it is known that a semiconductor magnetic sensor is used to detect a relative rotation angle of a rotor with respect to a stator. In this case, a secondary magnet is placed at a position corresponding to the semiconductor magnetic sensor. For example, in a brushless motor disclosed in JP-A-2006-081283, a secondary magnet is placed at an end of a shaft rotating with a rotor so that a semiconductor magnetic sensor can detect a relative rotation angle of the rotor without the influence of a magnetic field which is caused by electric current in a winding and a magnetic pole of the rotor.